1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
Much research is being carried out on a nonaqueous electrolyte secondary battery that is charged and discharged by the migration of lithium ions between the positive electrode and the negative electrode in an attempt to develop a battery having a high energy density.
In general, the nonaqueous electrolyte secondary battery, if over-charged, is accompanied by dangers such as temperature elevation, liquid leakage, gas release, and ignition. Therefore, a safety mechanism for decreasing or stopping the current is mounted to the nonaqueous electrolyte secondary battery.
The safety mechanism against over-charging is classified into an outer element and an inner element.
The outer element includes in general a PTC (Positive Temperature Coefficient) thermistor and a temperature fuse, which are of the temperature-dependent type. However, the outer element, which is detachable, lacks safety and gives rise to, for example, cost elevation and, thus, the outer element is not desirable. Also, the element of the temperature-dependent type was defective in that the operation thereof tended to be nonuniform.
Such being the situation, various inner elements of the pressure-dependent type, the expansion-dependent type, etc. have been proposed. However, in the case of employing the pressure-dependent type inner element, which is operated by the elevation of the internal pressure, it is impossible to use a flexible case made of a flexible material such as a laminate film. On the other hand, the expansion-dependent type inner element, which is operated by the expansion in the inner volume of the nonaqueous electrolyte secondary battery, makes it possible to use a flexible case. However, the operation of the expansion-dependent type inner element is made nonuniform by the size of the free space around the nonaqueous electrolyte secondary battery. It should be noted in particular that, where the free space around the nonaqueous electrolyte secondary battery is extremely small, the inner element is unlikely to be expanded, with the result that the safety of the nonaqueous electrolyte secondary battery is impaired.
On the other hand, Japanese Patent Disclosure (Kokai) No. 6-163052 discloses an alloying-dependent type inner element as a safety mechanism for the case where the nonaqueous electrolyte secondary battery is put under the reverse discharge state because the negative electrode potential is made nobler than the positive electrode potential after the over-discharge state. The inner element disclosed in this patent document comprises a positive electrode terminal formed of a metal that reacts with, for example, lithium ions under the reverse charging state so as to form an alloy, and a tensile stress imparting means for imparting a tensile stress to the positive electrode terminal. According to the inner element disclosed in this patent document, the positive electrode terminal is alloyed under the reverse charging state so as to be made brittle, with the result that the positive electrode terminal can be cut away by the tensile stress imparted by the stress imparting means. As a result, the current path is cut-off. It follows that the inner element disclosed in the patent document quoted above makes it possible to suppress the nonuniformity in the operation of the safety mechanism regardless of the material of the case of the battery.
However, the nonaqueous electrolyte secondary battery disclosed in the patent document quoted above gives rise to the problem that, if the cross sectional area of the positive electrode terminal is increased in an attempt to improve the high rate characteristics of the battery, the positive electrode terminal is rendered unlikely to be alloyed so as to impair the safety of the battery. Also, if the cross sectional area of the positive electrode terminal is decreased in an attempt to promote the alloying of the positive electrode terminal, an additional problem is generated that the high rate characteristics of the nonaqueous electrolyte secondary battery are lowered.